1. Field of the Invention
The present invention relates to an automatic gain control apparatus; more particularly, relates to an automatic gain control apparatus, which fully utilizes the input range of an A/D converter thereof.
2. Descriptions of the Related Art
An analog to digital (A/D) converter is frequently used in many kinds of electronic circuits. The function of an A/D converter is to receive an analog signal and to generate a digital signal in response to the analog signal.
In some kinds of electronic circuits, it is desirable to maintain a peak value of the analog signal smaller than a maximum value accepted by the A/D converter to avoid distortion. In some further kinds of electronic circuits, the peak value is deliberately arranged close to, but still smaller than, the maximum value so that the full input range of the A/D converter may be used. The A/D converter in an automatic gain control circuit is expected to meet the aforementioned requirements.
One example of the conventional automatic gain control circuit is disclosed in the U.S. Pat. No. 6,292,120. The automatic gain control circuit of the U.S. patent is illustrated in FIG. 1 of the present drawings. The automatic gain control circuit 1 includes an A/D converter 11, a fixed gain amplifier 12, a D/A converter 13, a microcontroller 15, and a variable gain amplifier 19. The microcontroller 15 is configured to generate a feedback signal 14 for the D/A converter 13 in response to a digital signal 16. The D/A converter 13 is configured to adjust a gain of the variable amplifier 19 so that an input range of the A/D converter 11 may be fully used.
One disadvantage of the automatic gain control circuit 1 is that this apparatus 1 cannot adjust a DC offset of an analog signal 10. If the DC offset is remained, the DC offset will influence the full utilization of input range of the A/D converter 11 because the DC offset shifts the center of the amplified analog signal from the center of the input range of the A/D converter 11. This makes that, for example, the top peak of the amplified analog signal reaches the upper boundary of the input range of the A/D converter 11, but the bottom peak of the amplified analog signal is not even close to the lower boundary of the input range if the DC offset is a positive voltage.
Another example of the conventional automatic gain control apparatus is disclosed in the U.S. Pat. No. 5,946,607. The automatic gain control circuit shown in FIG. 3 of the U.S. patent is illustrated in FIG. 2 of the present drawings. The automatic gain control circuit 2 comprises a gain controllable amplifier 21, an A/D converter 23, a DC offset detector 25, and an adder 27. The automatic gain control apparatus 2 generates a gain control signal 22 to control a gain of the gain controllable amplifier 21. A DC offset is detected at an output end of the A/D converter 23 by the DC offset detector 25. Then the DC offset is removed by the adder 27.
One disadvantage of the automatic gain control apparatus 2 is that though it removes the DC offset by the DC offset detector 25, the DC offset detector 25 operates after the A/D converter 23. This means that the DC offset still exists at an input end of the A/D converter 23. Hence, the DC offset still influences the idea of using the full input range of the A/D converter 23. Though the DC offset still can be removed by placing a high pass filter in front of the A/D converter, a required off-chip capacitor to achieve a low cut-off frequency value is costly.
Due to the drawbacks, an automatic gain control apparatus, which may fully utilize an input range of an A/D converter of an automatic gain control apparatus, is still needed in the industrial field. An automatic gain control apparatus, which may remove a DC offset in a more cost-effective way in front of the A/D converter, is more eagerly required.